Despite its vast economic and social impact, osteoarthritis (OA) remains poorly understood, especially the early asymptotic phase of the disease. The presently proposed study is intended to help delineate bone and mechanical factors associated with the progression of early asymptomatic radiographic hip OA as assessed by joint space narrowing rate. In this proposal, the applicants will test the hypothesis that proximal femoral bone mineral density (BMD) and hip joint loads during gait are positively correlated with the subsequent joint space narrowing rate. The hypothesis will be tested with data from the asymptomatic contralateral side of subjects receiving a unilateral total hip replacement (THR) for endstage primary hip OA. The applicants feel that this is an ideal population for testing this hypothesis, as they have shown that the joint space narrowing rate on the contralateral side of subjects with a unilateral THR has a bimodal distribution, with 15 percent of the subjects having an accelerated narrowing rate (greater than 0.2mm/year) and 85 percent of the subjects having a slow narrowing rate (less than 0.2mm/year). Moreover, in two separate pilot studies of the contralateral side of subjects receiving a unilateral THR for endstage OA, they have shown that elevated BMD is associated with a subsequent accelerated narrowing rate, and that the elevated hop joint loads during gait are similarly associated with an accelerated narrowing rate. Yet to be determined is the combined ability of BMD and hip joint loads to jointly predict the joint space-narrowing rat in this population. Given that bone is responsive to alterations in mechanical loading and that BMD and joint loading as assessed with gait analysis are correlated in endstage hip OA, it seems critical that the role of BMD and loading in progression of joint degeneration be evaluated concurrently. To determine the interrelationship between hip joint loads during gait and proximal femoral BMD, and to determine if either or both proximal femoral BMD and hip joint loads at baseline are positively correlated with the subsequent joint space narrowing rate, 162 subjects with an asymptomatic contralateral side will be enrolled at the time they receive a THR for unilateral primary OA. At the baseline evaluation, the proximal femoral and lumbar spine BMD will be determined from dual energy x-ray absorptiometry and the hip joint loads will be determined using gait analysis and an analytical parametric musculoskeletal model. The subsequent joint space narrowing rate will then be determined from the baseline radiograph and sequential radiographs taken over the next three years. Statistical models will be used to delineate the relationships among BMD, joint loads and the joint space narrowing rate.